Although glass is the most commonly used transparent material used as a glazing material in buildings, vehicles, and the like, glass is not acceptable for all such applications. Glass is heavy and brittle, and may shatter upon impact. Additionally, glass is difficult to form into complex shapes.
For many applications there exists a general need for mechanically strong composite materials that are of high optical quality and have an optical transparency similar or equivalent to that of ordinary window or plate glass. Polymers such as polymethylmethacrylate (PMMA), sold under the trademark Plexiglass.RTM., are often used in place of glass in certain applications in which both impact resistance and optical transparency are required. Unfortunately, polymers such as PMMA still do not have sufficient mechanical strength for many current applications.
Manufacturers of aircraft usually use such transparent polymers for the windows and canopies of aircraft. Because of the poor mechanical strength of such polymers, the polymeric side windows of aircraft are usually restricted to be of a very limited area. Windshields and canopies, which are required to be of greater surface area than side windows, must be made thicker and thus heavier than side windows, to provide sufficient mechanical strength. Still, such windshields and canopies are very vulnerable to impact, such as from a bird strike, which can, and sometimes does, result in breakage of the windshield and corresponding injury to the flight crew.
Thus, a specific need exists in the aircraft industry, as well as in many other industries, for a lightweight transparent material that possesses substantial strength, sufficient to withstand bird impacts. Such material must also be somewhat resilient so that it will flex without shattering when subjected to an impact concentrated in a small area and must be capable of being formed into the complex configurations typical of the canopies of some aircraft. A general need also exists for a method for increasing the mechanical strength and stiffness of PMMA and preformed structures made therewith, and with other such transparent polymers, while still retaining their good optical transparency.
One method of increasing the mechanical strength of polymers like PMMA is to reinforce them with strong cylindrical glass fibers ("glass fibers"). This technology is well known and is widely practiced in the manufacture of fiberglass reinforced plastics (FRP). In most cases, however, the introduction of glass fibers into an optically transparent polymer destroys the transparency of the polymer. All commercial FRP composites presently produced are either optically opaque or translucent such that an object at distances greater than about a few feet cannot be clearly seen through them.
In U.S. Pat. No. 5,039,566, the present inventors disclosed an FRP composite and a method of producing such a composite. The composite of that patent exhibits a high degree of optical transparency, much greater than any FRP commercially manufactured. This high degree of transparency was achieved by carefully matching the refractive index of the glass fibers with that of the polymer. By matching the refractive indices of the two materials across the visible spectrum, the scattering and reflection of light that normally occurs at the glass fiber/polymer interface, which causes the composite to become optically translucent-through opaque, is eliminated and the composite continues to be optically transparent.
One problem which exists in the aforesaid glass fiber/polymer composites is that changes in the temperature of such composites can cause the relative refractive indices of the glass fibers and polymer to change relative to one another such that they become mismatched. This mismatching results in a degradation in composite clarity. For example, when a glass fiber-reinforced PMMA composite is heated from 30.degree. C. to about 70.degree. C., the optical clarity is compromised. This lack of clarity with temperature change constitutes a primary limitation of glass fiber-reinforced transparent composites, i.e., they can only be used if they will be exposed to a narrow range of temperatures.
The foregoing limitations of glass fiber-reinforced composites is of particular significance in aircraft applications, wherein the temperature of the composite can very widely during a single flight, e.g., the composite is warm when leaving a warm, southern locale, becomes colder when reaching the upper atmosphere during flight, and colder still when the aircraft lands in a wintry northern city.
The method of making the product, which is also disclosed in U.S. Pat. No. 5,039,566, does present some difficulties. In accordance with the '566 method the transparent polymer, e.g., PMMA, is formed by polymerizing the appropriate monomer while glass fibers are maintained immersed within the monomer to produce a polymer matrix in which the fibers are embedded. While this process yields composites of improved mechanical properties (higher flexural strength, higher elastic modulus, and higher work of fracture), the polymerization requires a long time (several hours to days) and the resulting optical clarity is inadequate for certain applications.
Accordingly, a need still exists for a lightweight, highly transparent material that possesses substantial strength, which is capable of flexing without shattering when subjected to an impact concentrated in a small area and which is capable of being formed into the complex configurations typical of the canopies of some aircraft. A need further exists for a composite which is capable of being manufactured by a method that is less time-consuming than methods known in the art.
Further, a need exists for a method of increasing the mechanical strength of preformed structures made with PMMA and other transparent polymers. Such a method should employ only a minimum number of fibers, to retain as much as possible the optical clarity of the original structure.
In addition, there exists a need for a transparent material having enhanced mechanical strength whose optical clarity and distortion is relatively less sensitive to changes in temperature as compared with known materials of similar strength, e.g., its clarity remains substantially constant, or exhibits only a slight change, over a relatively broad range of temperatures.